The theory behind the behavior of the Rubens' Tube starts off simply. As we open the valve to our liquid propane, gaseous propane fills the tube and raises the interior pressure to just a bit above atmospheric pressure, depending upon the type of fuel source being used.
As the tube is lit we can see some indication of the flow rate through each hole by the flame height. To model this flow we can start with Bernoulli's Equation. Assuming inviscid and incompressible flow, and that the change in height of the fluid has a negligible effect on the energy balance, the governing equation for the velocity, V, shows that the velocity of the gas with density, ?, should be roughly proportional to the square root of the difference between the pressure in the tube, PT, and atmospheric pressure, P0.